SALT4 is a two-dimensional, analytical/displacement-discontinuity code designed to evaluate temperatures, deformation, and stresses associated with underground disposal of radioactive waste in bedded salt. SALT4 takes into account viscoelastic behavior in the pillars adjacent to excavations, transversely isotropic elastic moduli such as those exhibited by bedded or stratified rock, and excavation sequence. SALT4 can be used for parameter sensitivity analyses of two-dimensional, repository-scale, thermal and thermomechanical response in bedded salt during the excavation, operational, and post-closure phases. It is especially useful in evaluating alternative patterns and sequences of excavation or waste canister placement.

In SALT4, the temperature distribution and associated thermal stresses are approximated by analytic solutions for a line heat source in an elastic medium. The mechanical effects due to excavationof the repository openings are treated by the displacement-discontinuity method.

Although SALT4 was designed for analysis of bedded salt, it is also applicable to crystalline rock if the creep calculation is suppressed. The main disadvantage of SALT4 is that some of the assumptions made, i.e. temperature-independent material properties, render it unsuitable for canister-scale analysis or analysis of lateral deformation of the pillars.

The SALT4 model can serve as a valuable auxiliary to the more sophisticated SCEPTER finite-element and finite-difference thermal and thermomechanical codes by helping to resolve issues which arise from the finite calculational domain inherent in the latter approaches. The finite-element and finite-difference codes may require a very large number of nodes to circumvent effects of the finite domain inherent in their application. Because of the nature of the influence functions used in the SALT4 formulation, it models a semi-infinite domain and is therefore not subject to this limitations.